1. Field of the Invention
The present invention generally relates to a method and apparatus for monitoring the etching condition of a chemical etching process, and more particularly, to an improved contactless real-time in-situ method and apparatus for the same.
2. Discussion of the Related Art
Etching rates and etch end points must be carefully monitored and controlled in order to end etching processes at the desired time. In semiconductor processing, inadequate or excess etching time can result in undesirable film patterning. For instance, for semiconductor devices having film layers or features in the micron and sub-micron range, an inadequate etch or an excess etch would result in the insufficient removal or the excess removal of a desired layer. Insufficient removal of a desired layer can result in an undesired electrical open or electrical short when the desired layer to be removed is an insulator or a conductor, respectively. Additionally, if the etch is in excess, undercutting or punch through can occur resulting in poorly defined film patterning or total lift-off. Inadequate or excess etching time further leads to undesirable reliability problems in the subsequently fabricated semiconductor device. As a semiconductor wafer is extremely expensive due to many processing steps involved in the making thereof, the need to critically control the etching end point in an etching process is highly desirable.
An etch end point must be accurately predicted and/or detected to terminate etching abruptly. Etch rates, etch times, and etch end points are difficult to consistently predict due to lot-to-lot variations in film thickness and constitution, as well as etch bath temperature, flow, and concentration variability. That is, an etch rate is dependent upon a number of factors, which include, etchant concentration, etchant temperature, film thickness, and the film characteristics. Precise control of any of these factors can be very expensive to implement, for example, concentration control.
Film etching nonuniformity is decidedly disadvantageous in semiconductor processing. Where there is a spatially distributed film non-uniformity in a film to be etched, wafers must be overetched to completely etch the last-to-clear regions of the film. Thus there is necessarily a certain amount of overetching required. Non-uniformity can result from differences across the wafer in film thickness, or can result from differences in the physical or chemical properties of the film such as stoichiometry, density, or intrinsic stress. However, substantial overetching can lead to wafer yield loss and the decreased reliability of the resulting electronic devices. In addition, circuit dimensions must be made larger to allow for overetch tolerances. Therefore, uniform films are highly desirable in the manufacture of semiconductor devices. The optimal development environment for designing processes and processing tools that produce uniform films would have a quick, inexpensive, facile and accurate means of measuring total film uniformity. During the optimization of film deposition or growth, it is highly desirable to have quick feedback on the influence of process variables on the uniformity of the resulting film.
Currently, most etch rate end point determination techniques depend on indirect measurement and estimation techniques. Some etch monitoring techniques have relied on external measurements of film thickness followed by etch rate estimation and an extrapolated etch end point prediction. However, etch rates may vary due to batch-to-batch differences in the chemical and physical characteristics of the film or the etchant. These extrapolation methods are inadequate. Interrupted measurement techniques are also imprecise where the etch rate is not linear, such as where an induction period occurs at the beginning of the etch.
Previous methods for measuring film etching uniformity include optical techniques such as ellipsometry, reflectance spectroscopy, and the prism coupler method, on blanket films on monitor wafers. Film thicknesses measured on monitor wafers and even fiducial sites on product wafers do not always correlate to the actual film thicknesses in the region of interest (e.g. in contact holes, on stacks of films, etc.) in the device. These measurements are spatially discrete. They can be time-consuming especially when "complete" mapping of the thickness nonuniformity is needed to determine the maximum and minimum points across the film. Furthermore, optical measurements require expensive equipment and specialized training for unambiguous interpretation of results. They usually assume refractive index dispersion relations and optical constants of underlying films and substrates, which may be invalid. In addition, these techniques have limitations in the thickness ranges for which they are applicable.
Other methods include similar optical measurements of fiducial regions or discrete test wafers. However, such methods are expensive as portions of the wafer are occupied by non-product fiducial areas or require additional test wafers. Such optical methods are also subject to uncertainty resulting from turbidity of the etch bath and other optical effects and uncertainty resulting from non uniform films. Finally, such optical methods are subject to impression in the resulting estimate of overetch when the number of measured sites is insufficient or when the sections are not representative of the whole.
Real-time, in-situ monitoring is preferred. Some in-situ techniques monitor the etch rate of a reference thin film. This may require additional preparation of a monitor wafer containing the reference thin film or a suitable reference may be unavailable. Still other techniques require physical contact of electrical leads with the wafer being etched and electrical isolation of those leads and associated areas of the wafer from the etchant. This presents problems associated with contamination, contact reliability and reproducibility, and the physical constraints which affect ease of use in manufacturing or automation. Yet other in-situ techniques monitor the etch rate of a fiducial region of the product wafer and require optical access to the wafer in the wet etch bath. Such methods are expensive as portions of the wafer are occupied by non-product fiducial areas. Such optical methods are also subject to uncertainty resulting from turbidity of the etch bath and other optical effects and uncertainty resulting from non uniform films.
It would thus be desirable to provide an improved method and apparatus which provides non-contact, real-time, in-situ monitoring of an etching condition of a wafer being etched.